Protecting envelope for x-ray tubes



Feb. 10, 1959 w. BERENDS 2,

PROTECTING ENVELOPE FOR X-RAY TUBES Filed June 19, 1957 L-.. I I

| I l I INVENT'OR WERNER BERENDS PROTECTlNfi ENVELO?E FOR X-RAY TUBES Werner Berends, Hamburg, Germany, assignor to North American Philips Company, luc., New York, N. Y., a corporation of Delaware This invention relates to metal protecting-envelopes for use in X-ray tubes, the wall of which comprises a window of insulating material, for example casting resin or moulded insulating material, for the passage of the X-rays. It is customary to fill the envelope with oil and, in order for the layer of oil between the tube wall and the window to be thin the window is connected to the wall of the envelope by means of a re-entrant edge. If closure means are provided in front of the aperture for the passage of the rays, for example an ionisation chamber connected to the wall or the aperture is partly closed by a diaphragm limiting the beam, the air inside the closed space simulates an air-bubble in the oil filling. The air is strongly ionized by the X-rays and consequently has good electrical conductivity in comparison with the insulating material of the window and the oil-filling of the envelope. The influence thereof corresponds to that of a grounded electrode closely spaced from the glass wall of the X-ray tube. Due to the glass wall becoming charged, a high electric field strength may occur which produces a potential distribution over the window, the layer of oil and the glass wall, the voltage having to be taken to a considerable degree by the insulating material of the win dow. This gives rise to a spark discharge through the material.

The present invention provides a simple expedient for avoiding this detrimental phenomenon. According to the invention, the space enclosed by the window having a re-entrant edge and connected to the wall of the envelope is least partially filled with high-grade insulating material containing a large number of individual airbubbles.

The space may be filled up with granular material and may be shielded by means of a wall transparent to X-rays.

2,873,379 Patented Feb. 10, 1959 "ice Alternatively, the space may be filled with a material of foamy structure, a very suitable material being polystyrene. There is no objection to this elastic material protruding beyond the wall of the tube envelope, since the material is compressed when the envelope is pressed against a flat wall.

The marked result obtained by the invention may be explained by the fact that, although ionisation occurs in the space separated by the window from the interior of the envelope, this ionisation is limited to the air inside the bubbles, and the air-inclusions thus rendered conductve are insulated from one. another. As a re sult, the material used as a filler takes a part of the potential difference set up between the tube wall and the envelope of the tube, thus reducing the electrical load of the window, the layer of oil and the wall of the tube.

In order that the invention may be readily carried into effect an example will now be described in detail with reference to the accompanying drawing, which shows that portion of the envelope of the tube accord ing to the invention, which comprises a window 3 for the passage of the X-rays. The wall 1 of the envelope surrounds an X-ray tube 2 comprising a cathode 3 and an anode i. The envelope may be filled with oil. The window 5 for the passage of the X-rays is connected through the edge 6 to the wall 1 of the envelope. A space 7 bounded by the window and the edge is filled with a material 8 of foamy structure.

What is claimed is:

1. A protecting envelope for use in X-ray tubes, the

I wall of which comprises a window of insulating material for the passage of the rays, said window being connected with a re-entrant edge to the wall of the envelope defining therewith an enclosed space which is at least partially filled with high-grade insulating material contraining a great number of individual air-bubbles.

2. A protecting envelope as claimed in claim 1, in which the space is filled with insulating material of foamy structure.

3. A protecting envelope as claimed in claim 2, in which the insulating material consists of polystyrene and protrudes beyond the wall of the envelope.

References Cited in the file of this patent UNITED STATES PATENTS 

